The present disclosure generally relates to systems and methods for performing confocal thermoreflectance measurements, and more particularly, to confocal thermoreflectance imaging systems and methods that enable measurement of temperature distributions.
Improved thermal engineering can improve the operating characteristics and lifetimes of optoelectronic devices. For example, heating in semiconductor lasers can limit the maximum output power, shift the lasing wavelength, cause mode hopping, reduce lifetime, reduce the differential efficiency, increase the threshold current density, and limit the small signal modulation response. In addition, thermal stabilization of optoelectronic components is increasingly important to improving the performance of many photonic applications, such as wavelength division multiplexing and high-speed communications networks.
Furthermore, experimental exploration of the remarkably complex heat generation and transport processes in micro- or nano-structured optoelectronic devices such as diode lasers and semiconductor optical amplifiers is also challenging, in large part because the primary heat sources are often buried deep within these devices. Because of these difficulties, the thermal properties of optoelectronic devices are often thought of as bulk characteristics. For example, characterizations of semiconductor lasers such as optical spectra or power vs. current (LI) curves are often quoted at particular operating temperatures for the device as a whole, without detailed attention to the spatial heat distribution in the laser. These techniques are clearly insufficient when investigating nanostructured optoelectronic devices, since thermal variations occur on the submicrometer scale.
Thermoreflectance is a well-established non-contact method for measuring temperature distributions on a variety of different sample types. In the past decade, thermal imaging (as opposed to single point measurements) has become increasingly popular to measure surface temperature changes. As a result, different ways to achieve this goal have been published. Two-dimensional (“2-D”) stochastic-resonance enhanced thermoreflectance imaging has been previously demonstrated with 250 nanometer (“nm”) lateral spatial resolution and 10 milliKelvin (“mK”) thermal resolution.
Thermoreflectance microscopy exploits the change in reflectance R of a material with temperature T:
            Δ      ⁢                          ⁢      R        R    =                              1          R                ·                              ∂            R                                ∂            T                              ⁢      Δ      ⁢                          ⁢      T        ≡                  κ        ·        Δ            ⁢                          ⁢      T      by measuring small changes in the reflectivity ΔR, of a sample in response to temperature modulation ΔT. Typical values of the thermoreflectance calibration coefficient range from 10−6 Kelvin−1 (“K−1”) to 10−4 K−1, so lock-in techniques are required to extract the temperature signal. The prior single-point measurements and scanning techniques can be replaced by 2-D imaging onto diode-arrays connected to multiple lock-in amplifiers or charge-coupled devices (“CCDs”) with signal processing. However, until recently, it has been thought that the thermal resolution of imaging using a CCD is limited to 1 Kelvin (“K”) by the quantization limit of the camera. This high-resolution 2-D thermal imaging technique can investigate both the thermal behavior of a range of optoelectronic devices and also, in combination with a total energy balance model, characterize the optical power distribution within working photonic integrated circuits and other active devices. However, because current sub-micrometer-resolution thermal imaging techniques offer little depth resolution, they are limited to surface imaging, and therefore cannot be used to investigate heat flow deep within a device.
Heat transport in optoelectronic devices is known to be severely degraded by large numbers of epitaxial interfaces and by the use of alloyed materials. Early work on thin films and superlattices demonstrated strong anisotropy in in-plane versus cross-plane thermal conductivity. Molecular dynamics simulations of heat flow in heterostructures suggest that even a single interface can decrease cross-plane thermal conductivity κz by a factor of two; the presence of tensile strain further reduces κz. Furthermore, thermal conductivity can vary strongly with even small changes in material composition.
Thermal conductivity in superlattices is highly anisotropic and depends on a wide variety of factors, including interface quality, number of layers, layer thickness, lattice strain, and the ratio of the material composition. The cross-plane thermal conductivity κz can be reduced by up to a factor of 10 by phonon reflections at interfaces. Small reductions in the in-plane thermal conductivity κx also occur due to diffuse interface scattering. Results for GaAs/AlAs have shown that while the cross-plane thermal conductivity can be less even than the corresponding alloy value, the in-plane thermal conductivity of a GaAs/AlAs superlattice is usually less than that of the bulk materials but greater than that of the corresponding alloy.
In general, poor heat transport across heterojunctions results in relatively low thermal conductivities for complex optoelectronic devices. In particular, vertical cavity surface-emitting lasers (“VCSELs”) have a high thermal resistance due to their small size and the poor thermal conductivity of the mirrors (e.g., DBR mirror), so remarkably large variations (up to 200° C.) in the internal temperature distribution are predicted, both radially across the active region and vertically along the optical axis. In addition, prior work has shown radial surface temperature variations of up to 5 K between the center and edge of an operating VCSEL. Thermal models of edge-emitting lasers predict large variation in thermal impedance across the plane of the active region, resulting in temperature variations of up to 40%. Other work on quaternary blue-green lasers predicts temperature differences between surface and active region of 0.1-0.5 K for p-side up lasers and 1.5 K for p-side down devices.
A wide range of alternative methods for 2-D surface temperature measurements have been developed. A comparison of several temperature measurement techniques is found in FIG. 1, several of which are discussed below. Liquid crystal (“LC”) thermography provides good spatial and temperature resolution (1 micrometer (“μm”) and 0.05 to 0.5 K), but temperatures can only be measured relative to the clearing point temperature at which the crystals undergo a phase transition. Fluorescent microthermography is a similar thermal imaging technique with better temporal resolution; both of these methods require thin film deposition on the surface of the test device. Optical interferometry based on thermal expansion provides micrometer scale measurements with extremely good thermal resolution (10−6 K), but calibration of temperature based on surface displacement is very difficult for materials without a high thermal expansion coefficient. Scanning thermal microscopy can achieve a spatial resolution of 50 nm; this technique typically uses an atomic force microscope as a measurement platform.
The ability to measure temperature inside a three dimensional structure is currently very limited. Because Si and InP are transparent in infrared (“IR”) measurements (λ>2 μm), it is possible to use near-IR thermography to image flip-chip bonded ICs through the substrate; however, the lateral resolution is limited to 5 μm.
CCD thermoreflectance has been performed using the imaging optics of a widefield microscope, for which there is little depth discrimination and the Rayleigh criterion puts a lower limit on lateral spatial resolution of dx=0.6λ/NA where λ is the illuminating wavelength and NA is the numerical aperture of the microscope objective. Widefield microscopy has proven adequate for imaging the temperature distribution across the surface of a number of active optoelectronic devices, including semiconductor optical amplifiers, edge-emitting and surface-emitting diode lasers. However, accurate investigation of heat transport above and below dielectric layers (e.g., oxide passivation layers), across semiconductor interfaces (e.g., multi-quantum well active regions), or for devices with features less than 250 nm or alternatively offering poor image contrast demands further improvement in lateral and vertical spatial resolutions.
Therefore, there is a need for methods and systems for performing confocal thermoreflectance measurements that can measure temperature from reflective layers, objects, or defects, or for devices with features less than 250 nm or alternatively offering poor image contrast (e.g., devices such as transistors, nanocircuits, etc).
There is also a need for profiling thermal distribution both within an operating device, including LEDs, edge-emitting lasers, and VCSELs signal, and at the surface, heat sink, and sides, so that both the internal distribution and boundary conditions are understood.